I have a 21 gallon tank with a heater, filter, gravel, 2 live plants, 2 fake plants, 2 cave like looking hiding spots and a fake log looking ornament with a male betta in it. I'm currently trying to cycle the aquarium but it's taking too much time for the cycle to start and i was wondering what i could add with the betta to make it start faster!
Here's a picture of the tank.

If you're having a little trouble building up ammonia with just the betta, honestly, I would suggest removing him into a smaller QT and starting a fish-less cycle with pure ammonia. You could also try some bottled bacteria(a bit of a hit-or-miss, but I've heard some good things about Tetra Safe-start and Seachem Stability)then, see if that helps to jump-start things a little bit.

If you're really against the fish-less cycle, I'd suggest tossing in a snail or two. I wouldn't suggest adding any other fish until you get your tank cycled and stable.

If you're still interested in adding other tank mates after your tank is cycled....
Rasboras and Corys are both great Betta-compatable species, both very peaceful and great choices for tank mates. They are both schooling/shoaling fish however and need groups of 5-6+. Now, you could do about 6 of each(you'll want to add them slowly....a couple of fish ever few days-a week so you don't over-load your tank and crash your cycle), but personally I think it would be a little better to pick one species and just up the school/shoal number.

If you're still interested in adding other tank mates after your tank is cycled....
Rasboras and Corys are both great Betta-compatable species, both very peaceful and great choices for tank mates. They are both schooling/shoaling fish however and need groups of 5-6+. Now, you could do about 6 of each(you'll want to add them slowly....a couple of fish ever few days-a week so you don't over-load your tank and crash your cycle), but personally I think it would be a little better to pick one species and just up the school/shoal number.

Would it be bad to get both species? I want to get 6 cories and 6-8 rasaboras to make the tank more interesting and they would each have a shoal. Would they stress each other out or something?

No no, it wouldn't be bad. There is enough room for both species in a 20 if you keep both of the schools down around the minimum(6-ish, depending on filtration and plants you could get away with an extra one or two added to one of the schools), but of course schooling species always appreciate more of their own kind, and while a 20 gallon is a wonderful size with lots of options it is still not all THAT huge.

Its completely up to you though. You can do both species without any troubles, just as a personal opinion/preference one larger school is a little better then two smaller ones. ;)

I haven't heard much about Aquaplus I'm afraid, though I know not all bottled bacteria works well/at all. Even the brands I mentioned can be a hit or miss, but I have heard some good reviews on the Stability and Safe-start, so if you wanted to give the bottled bacteria a shot I'd suggest snagging one of those to try instead. From what I understand, it won't hurt to try.